This invention relates to sputtering systems, and more particularly to target movement for improved uniformity.
Thin films for electronic or optical devices can be deposited onto a substrate in a low-pressure sputtering chamber or machine. An ion-beam generator or source directs ions to the surface of a target, causing some target atoms or ions to be ejected from the target""s surface. These target ions then travel through the low-pressure chamber until reaching the surface of the substrate. A thin film of the target material, or a compound form of the target material, is deposited on the surface of the substrate.
Each target can be used many times, depositing films on many substrates that are loaded and unloaded into the chamber. Eventually the target becomes worn and must be replaced with a new target. Such target replacement may halt production runs and require a skilled technician. Minimizing target replacement is desirable; however, film quality is critical so targets are often replaced frequently to improve film uniformity.
FIG. 1A is a front view of a worn sputtering target. Repeated deposition runs using the same target results in removal of target material from target 10. Spot 12 is formed on the surface of target 10 where the ion beam impacts target 10. The size and shape of spot 12 can vary with the angle of the ion beam to target 10, and the type and beam-shape of ion-beam generator used. The location of spot 12 can be off-center of target 10 as shown.
FIG. 1B is a side view of a worn sputtering target. Depression 14 is formed on target 10 where spot 12 impacted the surface of target 10. The target material that was in depression 14 was ejected by the ion beam impacting the surface of target 10 in the region of depression 14.
More complex shapes of the ion-beam spot on the target are possible. FIGS. 2A, 2B show complex wear of a sputtering target. Spot 12 has several annular rings that are caused by the ion beam being somewhat non-uniform. Regions of the ion beam with a higher beam density or energy can sputter portions of target 10 at higher rates. Dimples 16 may be formed on the surface of target 10 where the beam has a higher energy or density. Dimples 16 may be ring-shaped or somewhat irregular.
FIG. 3A shows sputtering of a new target. Ion source 18 generates an ion beam that is directed onto target 10. Target ions or atoms are ejected from target 10 and travel to the surface of substrate 20, depositing a film on substrate 20. Substrate 20 can be removed and replaced with another substrate and deposition repeated many times with many different substrates before target 10 is replaced.
FIG. 3B shows sputtering of a worn target. After many depositions runs, dimples 16 have formed on the surface of target 10 where the ion beam from ion source 18 has impacted the surface. Since the surface of target 10 is no longer smooth but rough, some scattering of the ion beam can occur. The angle that target atoms or ions are ejected can vary with the surface roughness. Some ejected target atoms may not reach the surface of substrate 20, resulting in a lower deposition rate and possibly a less-uniform deposition across the surface of substrate 20. Since such scattering is undesirable, target 10 is often replaced more frequently to avoid such problems.
Since target replacement may require downtime of the sputtering machine, spare targets are sometimes included within the sputtering chamber to minimize such downtime. Target replacement can be automated, allowing 2 or 3 targets to be used up before all targets are replaced at one time.
FIG. 4A shows a top view of a sputtering chamber with multiple targets. Target 10 is mounted on target mount 22. Spare targets 15, 17 are also mounted on target mount 22. A motor can be activated to turn shaft 28, allowing a different or spare target 15 to be rotated into the position that was occupied by target 10, so that the ion beam from ion source 18 impacts spare target 15, which deposits a film onto substrate 20.
Shaft 28 can be rotated by one-third of a revolution, or 120 degrees, to exchange targets. Spare targets 15, 17 can be of the same target material as target 10, or can be of an entirely different target material. The sputtering machine can be programmed with a recipe that rotates shaft 28 to select one of the three targets for each deposition step. Multi-layer films can then be deposited.
FIG. 4B is a side view of a sputtering chamber with multiple targets. Ion-beam source 18 sends an ion beam that impacts target 10. Target atoms or ions ejected from target 10 travel to substrate 20 and are deposited on its surface. To improve film uniformity, substrate 20 can be rotated by a motor (not shown).
Spare target 15 (an another spare target hidden from view) and selected target 10 are mounted on target mount 22. Shaft 28 can rotate target mount 22 to select the desired target for sputtering. Motor 24 and gearbox 26 are activated to rotate shaft 28 by the desired amount. Motor 24 is mounted outside the low-pressure chamber 30 to minimize contamination.
Targets 10, 15 and substrate 20 can be heated or cooled as desired. Channels for cooling water can be formed on the backing plates to targets 10, 15. Motor 24 can also be operated during deposition to slightly rock target 10. Shaft 28 rocks back and forth by about 3 degrees to slightly vary the surface angle of the ion beam to target 10 during deposition. This rocking may improve uniformity.
Although target wear may be improved by such rocking, a mechanism to move the target is desired to improve uniformity. Rotation or spinning of targets has been used as one way to improve target wear. However, such target rotation may not sufficiently spread the wear over the target surface. A more aggressive way to move the target during deposition is thus desired to reduce target wear and improve uniformity of deposition.